Control method and apparatus for stabilizing optical wavelength

ABSTRACT

A control apparatus for stabilizing optical wavelength output by a laser module in which a laser element, temperature sensor and cooling/heating element are installed, comprises a temperature deviation detecting circuit comprising a temperature monitor for detecting a laser temperature, and a first comparator for outputting a first control signal indicating a difference between the laser temperature and a control target value, a wavelength deviation detecting circuit comprising a wavelength monitor for detecting light output from the laser element, and a second comparator for outputting a second control signal indicating a difference between the wavelength of the detected output light and a control target value, a selector circuit for selecting either of the detecting circuits according to the external conditions of the laser element, and a current controller for controlling the current supplied to the cooling/heating element based on the output signal from the selected detecting circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and is based on, and claimsthe benefit of priority from, Applicant's U.S. patent application Ser.No. 09/176,579, filed on Oct. 21, 1998, now issued as U.S. Pat. No.6,212,210, which in turn claims the benefit of priority from JapanesePatent Appl. No. 9-292228 filed on Oct. 24, 1997, both of which areincorporated by reference herein as filly as if set forth in theirentitety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a control apparatus and method for stabilizingoptical wavelength, and in particular a control apparatus and method forstabilizing optical wavelength which is suitable for use under anyexternal conditions.

2. Description of the Related Art

In recent years, with the development of multimedia communicationsservices, optical transmission systems which form the backbone ofcommunications systems are moving to higher speeds and highercapacities, and optical wavelength division multiplexing is expected tomake this high performance possible. In this optical wavelength divisionmultiplexing, several channels are transmitted on a common transmissionpath by assigning plural optical signals having different wavelengths ascarriers to plural signals which are to be transmitted. Therefore, toavoid inter-channel interference in optical wavelength divisionmultiplexing, interference between optical signals on adjacentwavelengths must be avoided, and optical wavelengths must consequentlybe stabilized with high precision.

Two factors that cause variation of the optical wavelength output by anoptical transmitter are, for example, the temperature variations oflaser elements and variation of laser diode driving current.

Conventionally, the optical wavelength output from the laser wasstabilized by for example controlling the temperature of the laserelement, or by monitoring the wavelength of the light output from thelaser element so as to control the laser element temperature, asdescribed hereafter.

First, referring to FIG. 1, a control mode will be described forstabilizing optical wavelength by maintaining the temperature of thelaser element constant (referred to hereafter as “constant temperaturecontrol” mode).

An optical transmitter shown in FIG. 1 comprises a laser module 4A inwhich a laser diode LD is installed together with a temperature sensor 5and a cooling/heating element 10, a temperature monitor 6 for monitoringa laser diode temperature using the temperature sensor 5, a target valuesetting circuit 8 for setting a target value of the laser diodetemperature, a comparator 7 for comparing a value S6 of lasertemperature monitored by the laser monitor 6 with a target value S8 setby the target value setting circuit 8, and a current controller 9 forcontrolling a current S9 supplied to the cooling/heating element 10based on a comparison result S7 in the comparator 7.

In the optical transmitter, the difference between the laser temperatureS6 monitored by the temperature sensor 5 and temperature monitor 6, andthe target value S8 set by the setting circuit 8, is detected by thecomparator 7, and sent to the current controller 9 as a deviation signalS7. In the current controller 9, an output current value is determinedso that the detected difference becomes 0, and the cooling/heatingelement 10 is driven by the determined current value. Due to thistemperature control, the temperature of the laser element (in this case,the laser diode) is kept constant, and the wavelength of light outputfrom the laser element is stabilized. A control method identical to thisis disclosed in Japanese Laid-open Patent Application No. 57-186383.

Next, referring to FIG. 2, a control mode will be described wherein theoptical wavelength of light output from the laser is monitored tostabilize the optical wavelength (referred to hereafter as “wavelengthmonitoring control” mode).

The optical transmitter shown in FIG. 2 comprises a laser module 4B inwhich a laser diode LD is installed together with the temperature sensor5 and cooling/heating element 10, optical coupler 11 for splitting partof the light output from the laser diode LD, optical wavelength monitor12 for receiving the split light and monitoring its wavelength, targetvalue setting circuit 13 for setting a target value of opticalwavelength, comparator 14 for detecting a difference between a value S12of optical wavelength monitored by the optical wavelength monitor andthe target value of optical wavelength set by the target value settingcircuit 13, and a current controller 9 for controlling the current S9 tobe supplied to the cooling/heating element 10 based on a deviationsignal S14 output by the comparator 14.

In the above optical transmitter, the wavelength of light output fromthe laser module 4 is monitored by the optical wavelength monitor 12,the difference between the monitored wavelength and the target value ofwavelength set by the setting circuit 13 is detected by the comparator14, and this difference is sent to the current controller 9. In thecurrent controller 9, the current value is determined so that thedetected difference becomes 0, and the cooling/heating element 10 isdriven by the determined current value. An identical control method isdescribed in Proceedings of the General Society of 1997 of the Instituteof Electronics, Information and Communication Engineers, B-10-215,p.724. In this control mode, as it is wavelength which is beingmonitored, wavelength variations due to laser element temperaturevariations or laser diode forward current variations can be suppressed.

In general, when a laser is operated for a long period, output powerfluctuations occur due to deterioration over time. For this reason, thelaser diode driving current is controlled to suppress output powerfluctuations by an Auto Power Control (APC) circuit, but the laseroptical wavelength also varies due to this change of laser diode drivingcurrent. Therefore, in the constant temperature control mode shown inFIG. 1, the optical wavelength of the laser varies as shown in FIG. 3together with time shown on the horizontal axis.

As shown in FIG. 3, when the laser operating time is short, a drivingcurrent IF of the laser diode LD may be considered constant. Withinthese limits, the output current from the current controller 9 iscontrolled to a fixed target value even if the laser diode temperatureS6 changes due to a variation of ambient temperature. Consequently, asthe cooling/heating element 10 (FIG. 1) is driven by this outputcurrent, the wavelength of optical output is stabilized in theshort-term.

However if the laser operating time is long, due to the laser'sdeterioration with time, the laser diode driving current IF(t) may varydue to the function of the APC circuit. In this case, a wavelengthfluctuation amount Δλ due to the laser diode driving current variationis given by the following equation (1).Δλ=α·{IF(tn)−IF(t 0)}  (1)

where α=conversion constant between laser diode driving current andwavelength variation, IF(t0)=laser diode driving current value atinitial time t0, IF(tn)=laser diode driving current value when time tnhas elapsed.

This wavelength fluctuation amount Δλ occurs regardless of laser diodetemperature, so it cannot be corrected in this control method.

Also, when laser temperature is feedback controlled to a constant value,there is generally a considerable delay in the response (temperaturevariations) to manipulations (cooling or heating), and if the feedbackgain is increased, the feedback loop may oscillate. Therefore, thefeedback gain must be made small, but as the control error is directlyproportional to the inverse of the feedback gain, the laser temperaturecannot necessarily be maintained constant to a high precision.Consequently, in this control method, it is difficult to achieve highlyprecise stabilized control of optical wavelength.

On the other hand, in the wavelength monitoring control mode shown inFIG. 2 where the laser temperature is controlled by monitoring theoptical wavelength of the light output from the laser, the opticalwavelength can be stabilized even if the laser diode driving currentvaries. Therefore, even over a long period of time when the laser maydeteriorate, the wavelength can be stabilized. Moreover, opticalwavelength can be monitored rapidly and with high precision, so in thiscontrol mode, the wavelength can be stabilized with high precision.

However, in the wavelength monitoring control mode of FIG. 2, when thewavelength monitoring value is 0 or has become unstable, the currentcontroller 9 recognizes a large difference between the opticalwavelength of the laser output and the control target value, andsupplies a current value to the cooling/heating element 10 toexcessively heat or cool the laser element. This may cause damage ordeterioration of the laser element.

Typical reasons why the wavelength monitoring value is zero or unstableare, for example, decrease of the split light led into the opticalmonitor 12, or instability of the optical wavelength output by the lasermodule 4. Therefore, for example, when a stop signal of the opticaloutput is supplied or a loss of the optical signal occurs in thetransmission path comprising the optical coupler 11 and wavelengthmonitor 12, or when an unstable wavelength state occurs immediatelyafter the source voltage is switched on, the signal value output by thewavelength monitor 12 may be 0 or unstable, and temperature control maybe performed leading to damage or deterioration of the laser element.

For example, when a stop signal SD is input as shown in FIG. 4, thedriving current IF of the laser element LD stops, and the optical outputfrom the laser decreases. In such a state, the output S12 of the opticalwavelength monitor 12 is apparently zero wavelength, the value of thewavelength difference output from the comparator 14 is a maximum value,and the output current value from the current controller 9 is also amaximum value. As a result, the laser module 4 is excessively heated orcooled by the cooling/heating element 10, and there is a risk that thismay lead to damage or deterioration of the laser element LD.

SUMMARY OF THE INVENTION

As described heretofore, in the constant temperature control mode, thereis a problem in that fluctuation of optical wavelength due to laserdiode driving current variation cannot be compensated. Further, in thecontrol mode where the optical wavelength value of the laser output ismonitored to control the temperature of the laser element, thewavelength monitor may become unstable depending on external conditions,and there is a risk that the laser may be damaged or sufferdeterioration.

It is therefore a first object of this invention to provide a controlapparatus and method for stabilizing optical wavelength which canstabilize an optical output wavelength to high precision over a longperiod of time regardless of external conditions under which the laserelement is controlled and operated.

It is a second object of this invention to provide an optical wavelengthdivision multiplexer which can avoid interference between wavelengths ofoptical signal channels regardless of external conditions.

To achieve the above objects, the control apparatus for stabilizingoptical wavelength of a laser output light according to the presentinvention comprises a plurality of control circuits for outputtingcontrol signals, in respectively different control modes, to controllaser element operation, and a selecting means for selecting one of saidcontrol circuits according to external conditions of the laser elementand applying a control signal output from the selected control circuit,thereby to perform stabilizing control of optical wavelength in theselected mode.

More specifically, to achieve the first object, the control apparatusfor stabilizing optical wavelength of light output by a laser accordingto the present invention comprises: a deviation detecting means fordetecting a first control deviation of one of a plurality of parameterswhich may cause a wavelength variation of laser output light from apredetermined control target value, an optical wavelength deviationdetecting means for detecting a second control deviation of thewavelength of laser output light from a predetermined target value, aselecting means for selecting one of the first and second controldeviations, and a control means for controlling one of the parameterssuch that the selected control deviation is reduced.

According to the present invention, the control apparatus forstabilizing optical wavelength of light output from a laser modulehaving a laser, temperature sensor and cooling/heating element,comprises a first control means for stabilizing the optical wavelength,a second control means for stabilizing the optical wavelength, aselecting means for selecting either of the first and second controlmeans according to external conditions, wherein the first control meanscomprises a temperature monitor for monitoring laser temperature by thetemperature sensor, a first comparator for detecting a differencebetween a laser temperature value output by the temperature monitor anda control target value, and a first current control means fordetermining a current value passed through the cooling/heating elementso that the difference detected by the aforesaid comparator becoms 0,and the second wavelength stabilizing control means comprises an opticalcoupler for splitting part of the output light from the laser module, anoptical wavelength monitor for monitoring the wavelength of the lightsplit by the optical coupler, a second comparator for detecting adifference between the value of the monitored optical wavelength and acontrol target value, and a second current control means for determininga current value passed through the cooling/heating element so that thedetected difference from the target value becomes 0.

To achieve the second object, the optical wavelength divisionmultiplexer for transmitting plural lights of different wavelengthaccording to this invention comprises a plurality of laser diodesinstalled on a common heat sink, a temperature controller forcontrolling the temperature of the heat sink to a predeterminedtemperature, an optical sensor for detecting respective oscillationwavelengths of the aforesaid laser diodes, driving current sources fordriving the laser diodes, a current controller for controlling thedriving currents respectively output by the driving current sources suchthat the oscillation wavelengths of the laser diodes approach targetwavelengths corresponding to each of the laser diodes, wherein thecurrent controller controls the driving current of one of the plurallaser diodes so that it approaches a predetermined target current if theoptical output of that laser diode shuts down, or if a predeterminedtime has not yet elapsed since optical output from that laser diodestarted.

The optical wavelength division multiplexer for transmitting plurallights of different wavelength according to this invention comprises aplurality of laser diodes, a plurality of driving current sources forrespectively driving the laser diodes, a plurality of optical sensorsfor respectively detecting the oscillation wavelengths of the laserdiodes, a plurality of temperature sensors for respectively detectingthe temperatures of the laser diodes, and a temperature controller forcontrolling the temperature of a laser diode so that the detectedoscillation wavelength approaches the target wavelength corresponding tothat laser diode when the oscillation wavelength of that laser diode isdetected to be stable, and controls the temperature of that laser diodeto a predetermined temperature for each laser diode when the oscillationwavelength of that laser diode is not detected to be stable.

According to this invention, output light having a stabilized opticalwavelength is obtained by selecting an optimum control mode suited todifferent conditions including short-term wavelength fluctuationsoccurring when a stop signal for the output light is supplied, whenthere is a loss of signal due to a detection circuit fault or when thesource voltage is switched on, and a long-term wavelength fluctuationdue to driving current increase when the laser deteriorates over time.

If the selectable control modes include a laser element constanttemperature control mode and a wavelength monitoring control mode,wavelength can be stabilized with high precision in the short-term byselecting the laser element constant temperature control mode. In thiscase, as control is performed regardless of the laser output light, itis effective when power is switched on, when a stop signal is input orwhen there is a loss of signal in the transmission path after thecoupler. On the other hand, by selecting the wavelength monitoringcontrol mode, control operations are carried out depend on thewavelength fluctuation of the output light. Therefore, both wavelengthfluctuations due to temperature variation of the laser element andwavelength fluctuations due to laser diode forward current variationsare suppressed, and high precision optical wavelength stabilization isachieved in the long-term.

If the source controller is shared, there is no need to provide aspecial driving controller for each control mode, and the scale of thecircuitry can be reduced. Also, if a delay means is provided to adjustthe control mode change-over timing, the control which is realized canbe adapted to the response speed of each control mode, and the responsespeed of the cooling/heating element.

According to the present invention, by arranging at least part of theoptical wavelength control functions in the optical transmission system,the optical transmitter can be made more compact. Also, if plural laserdevices are provided comprising optical wavelength stabilizing controlfunctions according to the present invention, a wavelength divisionmultiplexer can be obtained which avoids interference between laserwavelengths in adjacent channels in all cases regardless of externalconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a prior art control apparatus forstabilizing optical wavelength using a laser constant temperaturecontrol mode.

FIG. 2 is a block diagram showing a prior art control apparatus forstabilizing optical wavelength in a wavelength monitoring control mode.

FIG. 3 is a waveform diagram showing a relation between principalcontrol signals and output optical wavelengths in the prior art controlapparatus for stabilizing optical wavelength shown in FIG. 1.

FIG. 4 is a waveform diagram showing a relation between principalcontrol signals and output optical wavelengths in the prior art controlapparatus for stabilizing optical wavelength shown in FIG. 2.

FIG. 5 is a block diagram showing a first embodiment of the controlapparatus for stabilizing optical wavelength according to the presentinvention.

FIG. 6 is a waveform diagram showing a relation between principalcontrol signals and output optical wavelengths in the control apparatusfor stabilizing optical wavelength according to the present inventionshown in FIG. 5.

FIG. 7 is a block diagram of another embodiment of the control apparatusfor stabilizing optical wavelength according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention will now be described in detail referring to thedrawings.

First, a first embodiment of the invention will be described referringto FIGS. 5 and 6. According to this embodiment, two optical wavelengthcontrol circuits are provided with two different control modes, and afunction is provided to select one of these modes according to externalconditions.

The control apparatus for stabilizing optical wavelength shown in FIG. 5comprises a temperature deviation detector 110 for detecting a deviationof laser temperature from a control target value, a wavelength deviationdetector 120 for detecting a deviation of optical wavelength output bythe laser from a control target value, a selector 130 for determiningthe control mode to be performed according to external conditions, andselecting the appropriate deviation detector for the determined controlmode from either the temperature deviation detector 110 or thewavelength deviation detector 120, and a temperature manipulatorcontroller 140 for controlling the laser temperature according to adeviation signal S2 output from the deviation detector selected by theaforesaid selector 130.

The temperature deviation detector 110 detects a deviation of the laserelement temperature from a predetermined target value of lasertemperature, and outputs this as a deviation signal S7. The deviationsignal S7 is used in the control mode wherein the laser elementtemperature is maintained constant. The temperature deviation detector110 comprises a temperature sensor 5 installed together with a laserdiode LD in a laser module 4, a temperature monitor 6 which monitors thetemperature of the laser diode using the temperature sensor 5, a targetvalue setting circuit 8 which sets a target value of temperature towhich the temperature of the laser diode should be controlled, and acomparator 7 which compares the temperature value S6 monitored by thetemperature monitor 6 with the target value S8 set by the target valuesetting circuit 8, and outputs the result as the deviation signal S7.

The wavelength deviation detector 120 detects a difference of awavelength S12 of the laser output light from a predetermined wavelengthtarget value S13, and outputs this as a deviation signal S14. Thisdeviation signal S14 is used in the wavelength monitoring control modefor stabilizing optical wavelength wherein the optical wavelength λ ofthe light output from the laser is monitored. The wavelength deviationdetector 120 comprises an optical coupler 11 which splits part of thelight output from the laser diode LD in the laser module 4, an opticalwavelength monitor 12 which receives the split light and monitors itswavelength λ, a target value setting circuit 13 which sets a targetvalue S13 of wavelength, and a comparator 14 which compares the valueS12 of optical wavelength monitored by the optical wavelength monitor 12with the target value S13 set by the target value setting circuit 13. Asthe optical wavelength monitor 12, a wavelength meter or a wavelengthlocker comprising a band pass filter may for example be used.

The selector 130 determines the control mode according to the externalconditions of the laser, selects the deviation signal S7 or S14depending on the determined control mode, and supplies the selecteddeviation signal to the temperature manipulator controller 140. Theselector 130 comprises a control mode decision circuit 1 which readsplural state signals representing the external conditions and determinesthe control mode to be applied based on the external conditions, a delaytime circuit 3 which supplies a control mode signal S4 output by thecontrol mode decision circuit to a control mode selector circuit 2leaving a predetermined delay time taking account of a response timecharacteristic of the control mode and a response time of temperaturecontrol, and the control mode selector circuit 2 which selectivelysupplies either the deviation signal S7 or S14 to the temperaturemanipulator controller 140 according to a control mode signal S3 outputby the delay time circuit 3.

The delay time circuit 3 may for example comprise a low pass filter orthe like, and the control mode selector circuit 2 may for examplecomprise an analog switch or the like. The functions of the control modedecision circuit 1 and the delay time circuit 3 may be combined andimplemented by a microcomputer.

The temperature manipulator controller 140 controls the laser diodetemperature depending on the control mode selected by the selector 130.It comprises a cooling/heating element 10 installed on a substratetogether with the laser diode LD in the laser module 4, and a currentcontroller 9 for controlling the current supplied to the cooling/heatingelement 10 according to the deviation signal selected by the selector130.

Next, the operation of this control apparatus for stabilizing opticalwavelength according to the embodiment will be described.

First, operation in the constant laser temperature control mode, whichis one of the two available control modes, will be described. In thiscontrol mode, the laser temperature is monitored by the temperaturemonitor 6 using the temperature sensor 5 installed in the laser module4. The difference between the output value S6 from the temperaturemonitor 6 and the control target value S8 is detected by the comparator7. The current controller 9, by controlling the current value suppliedto the cooling/heating element 10 in the laser module 4, maintains thetemperature of the laser element LD constant such that this difference(signal S7) approaches 0. Therefore in this control mode, in theshort-term, high precision wavelength stabilization is achieved. As thelaser element temperature can be monitored regardless of the laseroutput light, control is stable even when power is switched on, when ashut-down signal is input, or when there is a loss of signal.

Next, operation in the wavelength monitoring control mode will bedescribed.

In this control mode, part of the output light from the laser module 4is split by the coupler 11, the wavelength of the split output light ismonitored by the optical wavelength monitor 12, and the differencebetween the value of the monitored light output wavelength and thecontrol target value 13 is detected by the comparator 14. The opticalwavelength of the output of the laser is stabilized by the currentcontroller 9, which controls the current value supplied to thecooling/heating element 10 in the laser module 4 so that the deviationsignal S14 output by the comparator 14 becomes 0. In this control mode,as the wavelength is monitored to control temperature, opticalwavelength variations due not only to laser element temperaturefluctuations but also to laser diode driving current fluctuations can besuppressed, and therefore, the optical wavelength can be stabilized to ahigh precision over a long period of time. However in such a case that,for example, power is switched on, a shut-down signal is input, or thereis a loss of signal after the coupler, as the monitoring of the opticalwavelength is difficult, there is a possibility of faulty operation.

In the control apparatus for stabilizing optical wavelength according tothe embodiment, when power is switched on, when a shut-down signal isinput or when there is a loss of signal in the optical signal path afterthe coupler, the constant laser temperature control mode is applied, andin all other cases, the wavelength monitoring control mode is applied.Hence, optical wavelength can be stabilized to high precision under anyexternal conditions.

Next, the control mode selection operation performed by the selector 130will be described referring to FIGS. 5 and 6.

In the high precision control apparatus for stabilizing opticalwavelength according to the invention, the control mode decision circuit1 reads parameters representing external conditions, and sets the outputsignal S4 to High or Low level according to these conditions. Aftersignals already received have been output for a certain time, the delaytime circuit 3 outputs the High/Low level signal S4 to the selectorcircuit 2. In this way, the control mode is changed over to suit theresponse time of each control mode or the response time of thecooling/heating element 10. When, for example, the input signal S4 isLow level, the control mode selector circuit 2 selects the differentialsignal S7 which is suitable for short-term wavelength fluctuations, andwhich is effective when power is switched on, when a stop signal issupplied, or when there is a loss of signal due to an obstacle in thetransmission path. Conversely, when the input signal S4 is High level,the control mode selector circuit 2 selects the differential signal S14of the wavelength monitoring control mode which is suitable forlong-term wavelength stabilization, and which is effective forwavelength fluctuations due to deterioration of the laser over time.

As parameters representing external conditions read by the control modedecision circuit 1, the state of the power supply (source voltage Vs),the presence or absence of a stop signal SD or of the state of theoptical output OPT may for example be used. The source voltage Vs andstop signal SD are led to the control mode decision circuit 1 via asignal line which respectively combines these signal sources. Theoptical output OPT can be detected using the output S12 of the opticalwavelength monitor 12.

When the states of these parameters are displayed by logical symbols asshown below, the output of the control mode decision circuit 1 may beexpressed by the logical equation (2).Control mode decision circuit output=SD+Vs+OPT  (2)Here,

-   -   Vs=1:source voltage ON, Vs=0:source voltage OFF,    -   SD=1:stop signal ON, SD=0:stop signal OFF,    -   OPT=1:optical output is normal, OPT=0:loss of signal.

FIG. 6 shows the variation of the external condition parameters Vs, SD,OPT, laser diode driving current IF, differential signal S7 of theconstant temperature control mode, differential signal S14 of thewavelength monitoring control mode, mode signal S4 output from thecontrol mode decision circuit 1, mode signal S3 output from the delaytime circuit 3, control mode selection signal in the control modeselector circuit 2, and the wavelength lambda of the laser output light,taking the elapsed time as the horizontal axis.

For example, when the stop signal is ON, the output of the control modedecision circuit 1 is Low level from the equation (2), and the controlmode selector circuit 2 selects the laser element constant temperaturecontrol mode which stabilizes the wavelength even when a stop signal isON.

For long-term stabilization control, the source voltage is ON, the stopsignal is OFF, and the optical output is normal. Therefore the outputsignal S4 of the control mode decision circuit 4 is High level from theequation (2), and the control mode selector circuit 2 selects thewavelength monitoring control mode which can stabilize wavelength in thelong-term, and which can compensate wavelength fluctuations due todeterioration of the laser over time.

According to the present invention, as the optimum control mode isautomatically selected as per equation (2) even when power is switchedon or there is a loss of signal, the optical output wavelength can bestabilized with high precision to suit external conditions. When thelaser element constant temperature control mode is selected, as highprecision wavelength stabilization can be achieved in the short-term,and as the temperature control signal is unaffected by the laser outputlight, abnormal operation of the control circuit can be avoided whenthere is a large variation of laser output light due to factors otherthan temperature variation, such as for example when power is switchedon, when a stop signal is supplied, or when there is a loss of signaldue to an obstacle in the laser light detection system comprising theoptical coupler 11. On the other hand, when the wavelength monitoringcontrol mode is selected, the wavelength is monitored, so wavelengthvariations due not only to laser element temperature variations but alsoto laser diode driving current variations can be stabilized with highprecision in the long-term.

Further, according to this embodiment, as the current controller 9 isshared between the two control modes, there is no need to provide aspecial current driving unit for each control mode, and therefore, thecircuitry can be made compact.

As the change-over timing of control modes is adjusted by the delay timecircuit 3, the control mode can be changed over to suit the responsespeed of each control mode and the response characteristics of thecooling/heating element 10. Therefore, if each of these control modeshas an optical wavelength stabilizing controller, plural opticaltransmitters having mutually different output wavelengths are providedand the outputs of these transmitters are combined into one by, forexample, an optical coupler, a stable optical wavelength divisionmultiplexer can be obtained with no inter-channel interference due towavelength variation.

Next, a second embodiment of this invention will be described referringto FIG. 7.

This embodiment differs from the first embodiment in that a constantforward current control mode is provided as one control mode instead ofconstant temperature control.

The control apparatus for stabilizing optical wavelength shown in FIG. 7comprises a current deviation detector 210 which detects a deviation oflaser diode driving current IF from a control target value and outputsthe detected deviation as a deviation signal S16, a wavelength deviationdetector 220 which detects a deviation of laser output light wavelengthfrom a control target value and outputs the detected deviation as adeviation signal S14, a selector 230 which determines the control modeaccording to external conditions and selects either the deviation signalS16 from the current deviation detector 210 or the deviation signal S14from the wavelength deviation detector 220, and a driving currentcontroller 15 which controls the laser diode driving current IFaccording to the deviation signal selected by the selector 230.

The current deviation detector 210 has a function of detecting adeviation of laser diode driving current from a predetermined controltarget value of laser diode forward current, and this current deviationis used for the control mode wherein the laser diode forward current ismaintained constant. The current deviation detector 210 comprises adriving current monitor 18 for monitoring the driving current IF of thelaser diode LD, a target value setting circuit 17 for setting a controltarget value I of laser diode driving current, and a comparator 16 forcomparing a value If′ of driving current monitored by the drivingcurrent monitor 18 with the target value I set by the target valuesetting circuit 17, and outputting a signal S16 showing the currentdeviation.

The wavelength deviation detector 220 detects a deviation of thewavelength of the light output by the laser from a predeterminedwavelength control target value, and this wavelength deviation is usedin the wavelength monitoring control mode. The wavelength deviationdetector 220 has an identical construction to that of the wavelengthdeviation detector 120 in the first embodiment described in FIG. 5, andits description will therefore be omitted here.

The selector 230 determines the control mode according to the externalconditions of the laser, selects a deviation according to this controlmode, and supplies this to the driving current controller 15. Theselector 230 has an identical construction to that of the selector 130in the first embodiment, and its description will therefore be omittedhere.

The driving current controller 15 controls the driving current IF fordriving the laser diode LD depending on the control mode selected by theselector 130.

Next, the operation of the optical wavelength stabilizing controlleraccording to this embodiment will be described.

First, operation in the constant laser diode driving current controlmode, which is one of the two available control modes, will bedescribed. In this control mode, the driving current IF of the laserdiode LD is monitored by the driving current monitor 18. A differencebetween the output value IF′ from the driving current monitor 18 and thecontrol target value I of driving current is detected by the detector16, and the current deviation signal S16 is output. In the drivingcurrent controller 15, the driving current value for driving the laserelement LD is controlled so that the difference between the presentdriving current shown by the deviation signal S16 and the target valueapproaches 0. In this way, the laser diode driving current IF ismaintained constant so as to coincide with the target value I, andshort-term, high precision optical wavelength stabilization is achieved.The laser diode driving current is monitored regardless of variations oflaser output light, so control is stable even when power is switched on,when a stop signal is supplied, or when there is a loss of signal due toan obstacle in the optical signal path after the coupler.

The operation in the wavelength monitoring control mode which is theother control mode of this embodiment is identical to that of the firstembodiment, and its description will therefore be omitted.

In the control apparatus for stabilizing optical wavelength according tothis embodiment, the constant laser diode driving current control modeis applied during periods when there is a large fluctuation of laseroutput light, such as when power is switched on or when a stop signal issupplied, and the wavelength monitoring control mode is applied at othertimes, hence the optical wavelength can be stabilized with highprecision under any external conditions.

In addition to the aforesaid two selectable wavelength stabilizingcontrol modes, the control apparatus for stabilizing optical wavelengthshown in FIG. 7 further comprises a temperature controller 250 formaintaining the laser temperature constant. By controlling the laser toconstant temperature and performing laser diode driving current controlavoiding optical wavelength fluctuations due to laser temperature, theoptical wavelength can be stabilized with high precision.

According to this embodiment, when the constant driving current controlmode is selected, short-term optical wavelength stabilization isachieved, and as the control signal is unrelated to the laser outputlight, this is effective when power is switched on, when a stop signalis supplied, or when there is an obstacle in the optical detection path.Further, when the wavelength monitoring control mode is selected, thelaser output is monitored, so wavelength fluctuations due to laserelement temperature variations or laser diode driving current variationsare suppressed, and high precision wavelength stabilization is achievedin the long-term.

By providing a plurality of optical transmitters having the opticalwavelength stabilizing control functions shown in FIG. 7, an opticalwavelength division multiplexer which avoids inter-wavelengthinterference between adjacent channels may be constructed. In this case,a plurality of lasers can be installed on a common heat sink substrate,and if a common temperature controller 250 is used, the opticalwavelength multiplexing transmitter can be made more compact overall.Moreover, even if heat flow blocks between heat sinks are omitted, theoptical wavelength can be stabilized with high precision.

In implementing this invention, when the optical transmitter comprisingthe laser module 4 is combined with an optical signal transmissionsystem comprising an optical fiber, part of the component elements ofthe control apparatus for stabilizing optical wavelength, for examplethe optical wavelength detector 120 and part of the mode selector 130,may be installed on the side of the optical signal transmission system,and the remaining component elements may be installed on the side of theoptical transmitter. The control apparatus and method according to thisinvention are also applicable to a modulator integrated type of lasermodule which includes integrated modulators for modulating light signalsoutput from the laser diodes.

1. An optical signal transmitter comprising: a laser diode for outputting an optical signal to be transmitted; a driving current source for driving said laser diode; a plurality of control circuits each providing a control signal for controlling the optical wavelength of said laser diode in different control modes, wherein each control circuit generates the control signal based on a mutually different control parameter corresponding to the control mode; and a selector to select one of said control modes according to the status of electrical signals representing external conditions of said laser diode, and to apply the control signal output from said selected control circuit to said laser diode driving current source, thereby achieving stabilizing control of optical wavelength with said selected control mode.
 2. An optical signal transmitter comprising: a laser module including a laser diode for outputting an optical signal to be transmitted and a cooling/heating element for adjusting the temperature of the laser diode; a first driving circuit for driving said laser diode; a second driving circuit for driving said cooling/heating element; a parameter deviation detector to detect a first control deviation of a parameter responsible for causing variations of optical wavelength output from said laser diode from a predetermined target value; an optical wavelength deviation detector to detect a second control deviation of optical wavelength output from said laser diode from a predetermined target value; and a selector connected to said detectors so as to select either of said first and second control deviations, wherein at least one of said first and second driving circuits is connected between said output of said selector and said laser module, to control said laser module so that said selected control deviation is reduced.
 3. An optical signal transmitter according to claim 2, wherein said parameter deviation detector detects the temperature of said laser module as said parameter.
 4. An optical signal transmitter according to claim 2, wherein said parameter deviation detector detects driving current of driving said laser diode as a parameter.
 5. An optical signal transmitter according to claim 2, wherein said selector is constructed so as to select said second control deviation when said second control deviation is stably detected by said optical wavelength deviation detector, and to select said first control deviation when said second control deviation is not stably detected.
 6. An optical signal transmitter comprising: a laser module including a laser element, a temperature sensor and a cooling/heating element; a first controller for stabilizing said optical wavelength; a second controller for stabilizing said optical wavelength; and a selector to select either of output signals from said first and second controllers according to the external conditions, so that stabilizing control of the optical wavelength of said laser element is performed according to the output signal from the selected controller, wherein: said first controller comprises a temperature monitor coupled with said temperature sensor to monitor the temperature of said laser element detected by the temperature sensor, a first comparator coupled with said temperature monitor to detect the difference between the output value of the temperature monitor and a laser temperature control target value, and a first current controller coupled with said cooling/heating element to control the current flowing in the cooling/heating element according to an output signal from said first comparator, and said second controller comprises an optical coupler arranged to split the output light from the laser module, an optical wavelength monitor coupled with said optical coupler to monitor the wavelength of the split output light, a second comparator coupled with said optical wavelength monitor to detect the difference between the monitored optical output wavelength value and a wavelength control target value, and a second current controller coupled with said cooling/heating element to control the current flowing in the cooling/heating element according to an output signal from said second comparator.
 7. An optical signal transmitter according to claim 6, wherein said first and second current controller comprise a common current controller connected to said first and second comparators through said selector.
 8. An optical signal transmitter according to claim 6, further comprising: a delay circuit coupled with said selector so as to delay the current control of said cooling/heating element based on said selected controller by a predetermined time after either of said first and second controllers to be selected is determined.
 9. A control apparatus for stabilizing the wavelength of light output from a laser module, comprising: a plurality of control circuits for outputting control signals to control the optical wavelength of said laser module in different control modes based on different control parameters to each other, a control mode decision circuit for generating a mode signal depending on external conditions of said laser module: and a selector for selecting one of control signals output from said control circuits according to the mode signal supplied from said control mode decision circuit, and applying the selected control signal to said laser module via a driving current source, thereby achieving stabilizing control of optical wavelength with one of said different control modes depending on the external conditions of said laser module.
 10. A control apparatus for stabilizing optical wavelength according to claim 9, wherein, first one of said control circuits outputs a control signal depending on a control deviation of optical wavelength output from said diode module from a predetermined target value, said control mode decision circuit generates, when said control deviation is stably detected by said first one of said control circuits, a mode signal for operating said selector to select the output of said first one of said control circuits, and said control mode decision circuit generates, when said control deviation is not stably detected, a mode signal for operating said selector to select the output of one of the other of said control circuits.
 11. A control method for stabilizing the wavelength of light output from a laser element, comprising the steps of: selecting at least one of a plurality of control circuits, to output a control signal for controlling the optical wavelength of said laser element in respectively different control modes according to the status of external conditions of said laser element, wherein each control mode is based on different control parameters representing external conditions detected by said control circuit that cause a wavelength variation, and applying a control signal output from said selected control circuit to said laser element, thereby achieving stabilizing control of optical wavelength with the control mode of said selected control circuit.
 12. A method for stabilizing optical wavelength according to claim 11, wherein in said selecting step, when said second control deviation is stably detected in said optical wavelength deviation detecting step, said second control deviation is selected, and when said second control deviation is not stably detected in said optical wavelength deviation detecting step, said first control deviation is selected. 